Some beam-beam effects during the intensity ramp-up 06/04/2012-08/04/2012

T. Pieloni with many inputs from: G. Arduini, X. Buffat, W. Herr, F. Roncarolo and the OP

crews on shift

• Fill 2480 and 2481 Intensities 1.6x1011 damped beams for LHCb wrong settings ( head-on collision instead of separated) over night 6/04-07/04

Fill 2482 STABLE BEAMS (1.3x1011 p/b) and 264 bunches with initial luminosity of >0.9x1033 cm-2 s-1 morning of 07/04

Fill 2488 and Fill 2489 STABLE BEAMS (1.3x1011 p/b) and 264 bunches leveling tests in IP1 and IP5 on 08/04

Summary of the fills

• Instability observed when going in collision (mostly vertical) but still far from head-on

Fill 2480 with 264 bunches (2480)

G. Arduini

• Squeeze without tune feedback. Significant reduction of the losses.

• Adjust:– Reduction of chromaticity before

going in collision (2 to 1 unit)– Increase of the damper gain in the

vertical plane• Still instability observed (mostly in

the H-plane)• Observed also head-on collisions

during the “rotation” of the LHCb crossing plane

Fill2481with 264 bunches (2481)

G. Arduini

• Sudden losses observed in some bunches. Patterns being verified (colliding in IP8)

Fills with 264 bunches (2482)

G. Arduini

The losses in details B1

Selected bunches start loosing after 2 hour of stable beams during leveling exercise of LHCb

Losses and filling scheme B1

The bunches loosing in B1 collide head-on only in IP8 while those in B2 collide also in

IP1 and 5…not clear evidence of causes from filling pattern but indication of LHCb

related…. still investigating!

Fills 2488 and 2489: Luminosity leveling with transverse offset

95% 75%

• Different dynamics of particles

• Orbit effects

• Slow emittance growth with offsets

• Passive compensation

IP1 & IP5 request reduced lumi to 70-80%

Beam-beam Effects

Beam-beam force

Why 95% and 75%

Different beam-beam dynamics at 0.4 s and 1.2 s

Depending on the offset the force seen by the other beam is differentDepending on the offset bunches have different tunes

d =

0 -

0.4

units

of b

eam

size

Orbit effects different due to pacman effects and the many long-range add up giving a non negligible effect

0.4 s offset is what the experiments will have from LR for nominal LHC

Small Offsets from long-range interactions

Why 95% and 75%

Ds /

s 0 (

per t

urn)

Maximum

Why 95% and 75%:Offsets in collision give Slow Emittance growth

LHC nominalDs

/ s

0

LHC

nom

inal

Ds /

s 0

And strong tune dependency stay away from 3rd order resonance

Reduce orbit effects (passive compensation)Interplay with parameters to not enter threshold values and working point optimization

Numerical models show a slow emittance growth (8% hour for nominal LHC)

Fill 2488 and 2489

Fill 2488 8th April 00:26-05:10ADT B1H ½ GAIN

Fill 2489 8th April 06:51-11:36ADT B1H back full GAIN @9:00

Bunch by Bunch losses Fill 2488 B2

IP8IP1&5IP1&5&8

ATLASCMSLHCB

LHCB leveling drives unstable few bunches of Beam 2Nothing to do with offset in IP1 & IP5

Bunch by Bunch losses Fill 2488 B1

IP8IP1&5IP1&5&8

ATLASCMSLHCB

Instability propagates to Beam 1 via coupling in IP1 and IP5

Few bunches of B1 & B2 unstable

During leveling at IP8 few bunches start loosing:

• B1: 2 bunches just colliding in IP8• B2: few bunches colliding in IP1-5-8

Instability starts with few bunches of B1 & B2 and then propagates to the other colliding partners at the different IPs

Beam 2

Beam 1

Special bunches?

These 4 bunches of Beam 2 Has full complement of LR collisions in IP8

Any coherent excitation on 1 beam couples to the other beam

Instability propagates to colliding pairs

Beam-beam couples the effect to many more bunches

Still investigating the source of transverse excitation

Beam 2

Beam 1

Tunes Fill 2488 B1

Clear evidence of beam-beam tune shifts during the leveling test but still analysis needed

B1 V B2 V

Bunch by bunch Emittances B1 Hor Fill 2488

75%100%

100%75%

No significant/obvious effects due to offset at IPsDetailed analysis still on-going to relate to bunch collision schedule and tunes

Bunch by Bunch losses Fill 2489 B1

Beam 1 during 95% luminosity test is stable and no visible differences in lifetimes

IP8IP1&5IP1&5&8

ATLASCMSLHCB

Bunch by Bunch losses Fill 2489 B2

Beam 2 during 95% luminosity test stable and no visible differences in lifetimes

IP8IP1&5IP1&5&8

ATLASCMSLHCB

Tunes Fill 2489

95% leveling almost zero effect on tunes but still data to be analyzed

B1 H B2 H

Bunch by bunch Emittances B2 VER Fill 2489

95%100%

100%95%

No significant effect related to levelingDetailed analysis still on-going to relate to bunch collision schedule

• Intense Easter period….• Some features of sudden losses on selective bunches still to be understood but seems to appear often after hours of stable beams •Successful test with static offsets with 75% and 95% luminosity

• No visible problems from beam-beam for the test configuration (emittances, lifetimes)

• Any movement of one beam (leveling) is transferred to the colliding bunches

• Detailed analysis on-going and some observations still to be understood•detailed analysis on-going

• Still many fills to analyze (CMS separated at 2.5 s Fill 2505 and Fill2509)

Summary